1. Field of the Invention
This invention relates to the field of analog to digital converters, and particularly to a second order Sigma-Delta based converter having superior analog components, and having a programmable comb filter which is coupled to the digital signal processor.
2. Background Art
Most analog to digital (A/D) converters can be classified into two categories, Nyquist-rate converters and oversampling converters. Nyquist-rate converters sample the analog input signal at the Nyquist frequency, which is slightly more than twice the maximum frequency of the input signal. Oversampling converters, on the other hand, perform the sampling at a frequency that is several times greater than the Nyquist frequency.
For several reasons, the popularity of a particular oversampling converter, the Sigma-Delta based converter, has increased recently. One reason for this recent surge in the popularity of these converters is the compatibility of the Sigma-Delta based converters with VLSI technology. More precisely, Sigma-Delta based converters provide for monolithic integration of both the analog and digital sections on a single die, since Sigma-Delta based converters utilize digital filtering techniques and thus have a high percentage (almost 90%) of digital circuitry. Also, the popularity of Sigma-Delta based converters has increased because they take advantage of the exceptional high speeds achieved with VLSI technology. In other words, these oversampling converters trade the excess resolution in time for improved resolution in amplitude. In addition, the popularity of Sigma-Delta based converters is due to their simple anti-aliasing requirements. A Sigma-Delta based converter does not require an expensive anti-aliasing filter, since the transition band of an anti-aliasing filter of an oversampling A/D converter is much wider than its pass band, and since the complexity of the filter is a strong function of the ratio of the width of the passband to the width of the transition band.
A Sigma-Delta based analog-to-digital converter has a high resolution and a large dynamic range since it utilizes oversampling, noise shaping, and digital filtering techniques. More specifically, this type of converter uses a Sigma-Delta modulator which, by oversampling and coarsely quantizing the analog input signal, moves the quantization noise energy out of the baseband and toward the higher frequencies. For example, in order for a Sigma-Delta modulator to provide 16 bits of resolution, it must attenuate the baseband quantization noise by 96 decibels (i.e. it must have a dynamic range of 96 dB). The digital output of the modulator is then provided to a decimator, which acts as a lowpass digital filter by computing a more precise estimate for the analog input at a lower sampling rate. Consequently, the high resolution and large dynamic range of a Sigma-Delta based converter are attributable (1) to the Sigma-Delta modulator reducing the baseband quantization noise energy by oversampling the input signal, and (2) to the decimator filtering out the high-frequency quantization noise by computing a more accurate estimate of the analog input at a lower sampling rate.